1. Field of the Invention
This invention relates generally to electrical storage batteries and more particularly to an improved sealed nickel cadmium battery cell which is immune to excessive hydrogen pressure buildup during overdischarge of the cell.
2. Prior Art
Nickel cadmium batteries are widely used for a variety of electrical power storage applications, notably electrical power storage in spacecraft. The basic arrangement, construction, and chemistry of such batteries are well known and hence need not be elaborated on in this disclosure.
Suffice it to say that sealed nickel cadmium batteries of the kind used for spacecraft energy storage, while having many desirable features and advantages for this type of service, have an outstanding disadvantage or deficiency which detracts from their usefulness, particularly for orbiting satellite applications. The deficiency referred to resides in the fact that in the absence of any preventative or corrective measures, it is possible for some battery cells to be forced into a voltage reversal or overdischarge condition during orbital cycling of the batteries due to cell imbalance. This condition, if it occurs, results in hydrogen evolution at the positive battery electrodes which, if not compensated for, will result in an excessive pressure buildup in the sealed batteries and eventual catastrophic failure of the batteries if not the entire satellite.
At the present time, this problem is alleviated by the use of protective bypass electronics for each cell which operates to bypass current around the cell if and when cell voltage reversal occurs. This type of protective system, while perhaps satisfactory from the standpoint of cell and battery protection, is costly, adds substantial weight to the battery package, and hence is not totally satisfactory.